Most FPGAs that are viable accellerators aren't for hobbyists but they also are not as expensive as you think. I can't find it anymore but I once saw an online shop with a huge variety of 500k+ LUT FPGA modules (just the chip itself on a small PCB) for around 1000€ + 500€ breakoutboard/mainboard. At those prices it makes more sense as an individual to invest into more CPU cores or a GPU (if your problem maps to it).

Edit: Maybe its this one here. https://shop.trenz-electronic.de/de/TE0808-04-06EG-1EE-Ultra...